An air flow measuring device that measures an intake air amount of an engine is conventionally known. This air flow measuring device includes a bypass passage that takes in a part of air flowing in an air intake duct, and a flow rate detecting element is disposed in this bypass passage. When the flow of air flowing into the bypass passage has a biased flow speed distribution, a turbulence is produced in the flow of air flowing through the flow rate detecting element. Thus, there arises the issue of reduction of the detection accuracy of the flow rate detecting element, which makes its output characteristics unstable. JP2014-1954A discloses the technique for restraining a turbulence of air by providing a reduced part in a bypass passage to reduce a cross-sectional area. The reduced part has the maximum reduction part at which the cross-sectional area of the bypass passage is the smallest, and a return surface that gradually increases the cross-sectional area is formed toward a downstream side of this maximum reduction part in the air flow direction.
However, the reduced part described in JP2014-1954A includes the flat surface perpendicular to the wall surface of the bypass passage at the downstream end of the return surface. Thus, the cross-sectional area of the bypass passage rapidly increases at the downstream end of the return surface. A vortex is accordingly generated in the region where the cross-sectional area rapidly increases, and the vortex reaching a flow rate detecting element at the time of a backflow may reduce the detection accuracy of the flow rate detecting element. The backflow means an air flow in the backward direction produced in the bypass passage due to the suction pulse of the engine.